Weak dependence on psutil

Description

pyutilib.th is configured to require psutil, but the nose plugin using is setup to conditionally require psutil.

Thus, I'm considering making pyutilib.th weakly dependent on psutil. The motivation is that psutil installation requires a C-level build that fails on MacOS. Since this isn't core functionality for Coopr/PyUtilib? users, but is only used to support testing, this weak dependence makes sense to me.